Bindo brings Beatrice to meet Mandragola in the woods in order to conjure the presence of Castruccio (386-390). This is where Beatrice last encounters Tripalda, which sends her into the final bout of madness that leads to her death. She does not die, however, until she after she is returned by Castruccio's orders to Euthanasia's palace in Lucca (391-3) .
I based my approximation of the location of these woods on the following passage: "[T]he Albinois brought a message from Mandragola, bidding Beatrice repair that same night at twelve, to a wood about four miles from Lucca. At this time Castruccio was employed in building the tower of Nozzano, on a small hill which was surrounded by the wood chosen by the witch. ... Castruccio was accustomed to repair, as early as two in the morning, to visit the fortress. Mandragola knew the spot near which he passed; and she so planned her scheme, that her incantation should be wound to the desired pitch, at the moment when he should ride through the wood" (386). The fortress to which Shelley refers here is probably the Castello Di Nozzano, which was "built by the Lucca inhabitants between the 12th and 13th centuries. ... Destroyed by the Pisans for the first time in 1314, it was rebuilt a few years later by Castruccio Castracane" (Lucca 5 – Turismo – Nozzano – Vacanze - my translation, with help of Google Translate). I marked the pin at a spot about four miles away from Lucca on the way to the Castello Di Nozzano.

Castello Di Nozzano